The role of the PMN oxidative burst in Fc-mediated phagocytosis: Phagocytosis of selectively opsonized sheep erythrocytes (E) was studied. CGD and MPO deficient PMN were found to have markedly enhanced phagocytosis when compared with normal PMN. This enhanced activity apparently relates to deficiencies in oxidative burst in these cells. Phagocytosis by normal PMN was examined after treatment of the cells with agents known to block various steps of the oxidative burst. Sodium azide and cyanide, which block MPO, markedly enhanced phagocytosis by normal PMN. Scavengers of H2O2 (catalase), also enhanced phagocytosis, but not to the degree of sodium azide. Our studies indicate that the late acting products of the oxidative burst, produced by the interaction of the MPO-H2O2-Halide system, are primarily responsible for the abrogation of ingestion seen in normal PMN. Binding and ingestion of IgG, but not C3b-coated particles, appears to be regulated by this system. Complement receptor function in degradation of C3b: Neoplastic and normal human B cell lines were examined for their ability to degrade bound C3b. Raji cells, which do not express the C3b receptor, CR1, mediated C3b cleavage in the presence of the enzyme, Factor I. A series of studies was performed comparing the activity of Raji cells with cell lines derived from normal B cells and with human erythrocytes, which express only CR1. Inhibitors of Factor H (a known cofactor for 1 activity) and monoclonal anti-CR1 and anti-CR2 (C3d receptor antibody) did not block the activity of the B cell lines. Anti-CR1 completely blocked C3b cleavage by human E. This suggests that, in addition to CR1, other receptors or membrane factors on B cells participate in the physiologic degradation of C3b.